1. Field of the Invention
The present invention relates to branching filters including transmitting filter chips and receiving filter chips, and more particularly, to a branching filter including a receiving filter chip and a transmitting filter chip defined by acoustic wave chips and a method for manufacturing the branching filter.
2. Description of the Related Art
For mobile communication devices, such as mobile phones, various branching filters including receiving bandpass filters and transmitting bandpass filters have been proposed. For example, Japanese Unexamined Patent Application Publication No. 2003-517239 discloses a branching filter including a receiving bandpass filter and a transmitting bandpass filter each including a surface acoustic wave filter. In Japanese Unexamined Patent Application Publication No. 2003-517239, the surface acoustic wave filters, which are included in the receiving and transmitting bandpass filters, are disposed on a common piezoelectric substrate. Since the receiving and transmitting bandpass filters are disposed on the common piezoelectric substrate, the branching filter has a relatively small size.
However, receiving bandpass filters and transmitting bandpass filters have different pass bands from each other and therefore have different requirements from each other. Therefore, Japanese Unexamined Patent Application Publication No. 2003-517239 further discloses a structure in which a surface acoustic wave filter included in a transmitting bandpass filter and a surface acoustic wave filter included in a transmitting bandpass filter are disposed on different piezoelectric substrates.
On the other hand, Japanese Unexamined Patent Application Publication No. 11-26623 discloses a structure in which a first surface acoustic wave filter chip defining a transmitting bandpass filter and a second surface acoustic wave filter chip defining a receiving bandpass filter are mounted on a mounting board by a face-down process. FIG. 11 is a schematic front sectional view of a branching filter 501, disclosed in Japanese Unexamined Patent Application Publication No. 2003-517239, including a mounting board 502. The mounting board 502 supports a first surface acoustic wave filter chip 503 and second surface acoustic wave filter chip 504 mounted thereon with a plurality of metal bumps 505 and a plurality of metal bumps 506, respectively, by a face-down technique. The first surface acoustic wave filter chip 503 is overlaid with a heat-dissipating member 507. The upper surface of the heat-dissipating member 507 is in contact with a shielding member 508 made of metal.
In the branching filter 501, the first surface acoustic wave filter chip 503 is different from the second surface acoustic wave filter chip 504. A piezoelectric substrate included in the first surface acoustic wave filter chip 503 and a piezoelectric substrate included in the second surface acoustic wave filter chip 504 can be selected depending on required frequency characteristics.
In the branching filter 501, an electrode including an IDT is disposed under the lower surfaces of the piezoelectric substrates, which are included in the surface acoustic wave filter chips 503 and 504. The electrode faces a space defined by the thickness metal bumps 505 and 506. Therefore, surface acoustic waves are not prevented from being excited.
Japanese Unexamined Patent Application Publication No. 2003-37471 discloses an example of a surface acoustic wave device, different from a branching filter, having a space provided so as not to prevent the excitation of surface acoustic waves. FIG. 12 shows a surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2003-517239 in front sectional view. The surface acoustic wave device 521 includes a surface acoustic wave element 524 including a piezoelectric substrate 522 and an electrode including IDTs 523 arranged on the lower surface of the piezoelectric substrate 522. The surface acoustic wave element 524 is bonded to a resin substrate 525 with a cured thermosetting resin 526a and a cured photocurable resin 526b such that the IDTs 523 are arranged on the lower surface thereof. After the lower surface of the surface acoustic wave element 524 and the upper surface of the resin substrate 525 are coated with an uncured thermosetting resin composition and an uncured photocurable resin composition and then attached to each other as shown in FIG. 12, the compositions are cured by applying heat and light thereto. This allows the surface acoustic wave element 524 and the resin substrate 525 to be bonded to each other with the cured products 526a and the 526b. A space A corresponding to the thickness of the cured products 526a and the 526b is present therebetween.
Japanese Unexamined Patent Application Publication No. 2003-517239 discloses a branching filter including a transmitting bandpass filter and receiving bandpass filter that are individually formed. Since the transmitting and receiving bandpass filters are individually formed as surface acoustic wave filter element chips, there is a problem in that the branching filter has a large size.
In the branching filter 501 disclosed in Japanese Unexamined Patent Application Publication No. 11-26623, the first and second surface acoustic wave filter chips 503 and 504 are mounted above the mounting board 502 by the face-down process. Therefore, the size of the branching filter 501 can be reduced.
In the branching filter 501, the first and second surface acoustic wave filter chips 503 and 504, which include the different piezoelectric substrates, are spaced from each other at a predetermined distance as shown in FIG. 11. The first and second surface acoustic wave filter chips 503 and 504 are mounted above the mounting board 502 with the metal bumps 505 and 506. Therefore, the areas of the piezoelectric substrates, which define the first and second surface acoustic wave filter chips 503 and 504, are relatively large. Therefore, an area of the branching filter 501 is relatively large.
Since the space can be formed depending on the height of the metal bumps 505 and 506, the branching filter 501 can be manufactured so as to have a reduced height. However, the height reduction thereof is limited because the metal bumps 505 and 506 have a relatively large height.
In the surface acoustic wave device 521 disclosed in Japanese Unexamined Patent Application Publication No. 2003-37471, the surface acoustic wave element 524 is bonded to the resin substrate 525 with the cured products 526a and 526b and the space A corresponding to the thickness of the cured products 526a and the 526b is present therebetween. Thus, the surface acoustic wave device 521 has a reduced height. However, the surface acoustic wave device 521 includes the surface acoustic wave element 524, in which the IDTs 523 are arranged on the single piezoelectric substrate. Therefore, a branching filter that includes a transmitting bandpass filter including the surface acoustic wave device 521 and a receiving bandpass filter including a similar surface acoustic wave device including another piezoelectric substrate cannot avoid having a relatively large size.
Recently, various boundary acoustic wave filter devices including IDTs disposed between piezoelectric substances and insulating substances have been proposed. The boundary acoustic wave filter devices do not require any space for not preventing vibration and therefore can have a reduced height. However, the boundary acoustic wave filter devices are disadvantageously inferior to surface acoustic wave filter devices in terms of power resistance. Therefore, although the boundary acoustic wave filter devices can be used for receiving bandpass filters for branching filters, surface acoustic wave filter devices are required to be used for transmitting bandpass filters. If a boundary acoustic wave filter device is used in a filter element chip included in a branching filter, the degree of height reduction is limited.